Method for when necessary automatically limiting a pressure in a hydraulic system during operation

ABSTRACT

A method is provided for, when necessary, automatically limiting a pressure in a hydraulic system during operation. A system is arranged to deliver pressurized hydraulic fluid to at least one fluid actuated device arranged to perform a work function, where the procedure includes sensing a pressure in at least one position of the system, comparing the detected pressure value, or an associated value, with a first predefined pressure limit, and opening a communication of fluid between the fluid actuated device and a reservoir through a first conduit if the sensed pressure value, or an associated value, exceeds the predefined limit.

BACKGROUND AND SUMMARY

The present invention relates to a method for automatically limiting apressure generated during operation in a hydraulic system when needed,said system being adapted to deliver a pressurized hydraulic fluid to atleast one actuator adapted to perform a work function.

Below, the invention will be described in connection with a workingmachine in the form of a wheel loader. This is a preferred, but by nomeans limiting application of the invention. The invention can forexample also be used for other types of working machines (or workvehicles), such as a backhoe loader, an excavator, or an agriculturalmachine such as a tractor.

A wheel loader can be utilised for a number of fields of activity, suchas lifting and transportation of rock and gravel, loading pallets andlogs. In each of these activities, different equipment is used,including implements in the form of a bucket, a fork implement andgripping arms. More particularly, the equipment comprises a load-armunit, or boom, which is pivotally arranged relative to the wheel loaderframe. Two actuators in the form of hydraulic cylinders are arrangedbetween the frame and the load-arm unit in order to achieve a liftingand lowering movement of the load-arm unit. The implement is pivotallyarranged on the load-arm unit. An additional actuator in the form of ahydraulic cylinder is arranged between the implement and the load-armunit in order to achieve a tilting movement of the implement.

The hydraulic system comprises a pump adapted to supply the hydrauliccylinders with pressurized hydraulic fluid via a hydraulic circuitcomprising a plurality of control valves.

As a rule, a wheel loader has more hydraulic functions than theabove-mentioned lift and tilt function. Such additional hydraulicfunctions include steering, 3rd, 4th, and in some cases even morefunctions. Each function generally needs two shock valves, except liftwhich has one shock valve. For a machine with a 3rd and a 4th function,this implies nine shock valves.

Different functions require different flow rates. Furthermore, the samefunction requires different flow rates for piston and piston rod side.Machines of different sizes also have different flow rate requirements.In practice, only a few shock valves are used, where the one having thehighest flow requirement decides the flow rate. This implies that mostfunctions have unnecessarily large shock valves.

It is desirable to achieve a method which creates prerequisites for amore cost efficient system with maintained or improved service life.

A method according to an aspect of the present invention includes

-   -   detecting a pressure in at least one position in the        system;—comparing the detected pressure value, or a value        associated with the detected pressure value, with a first        predetermined limit value; and    -   opening a flow communication between the actuator and a tank via        a first conduit if the detected pressure value, or the value        associated with the detected pressure value, exceeds the        predetermined limit value.

Thus, in this way, drainage to tank is actively controlled when needed.Preferably, at least one pressure sensor is adapted to detect thepressure to the respective function.

In this way, the limit value (for example the opening pressure) can beset as low as possible in all situations, which results in a smallerload on the constituent components.

According to a preferred embodiment, the flow communication is openedvia a control valve being arranged on the first conduit and having thefunction to control the supply of the hydraulic fluid to and from,respectively, the actuator with the object of performing the workfunction. In case of an unexpected pressure increase, this control valvefunctions as a controlled shock valve. Preferably, separate inlet andoutlet valves to the actuator are provided in order to control thefunction (for example a lifting and lowering movement).

According to another preferred embodiment, the method further comprisesthe step of opening a flow communication between the actuator and thetank via a second conduit via a shock valve. The shock valve is alsocalled pressure limiting valve. The shock valve is preferably arrangedin a conventional way as a passive (directly controlled by thepressure), for example spring-loaded, shock valve. By means of combiningthe opening of the control valve and the shock valve, drainage to tankat a desired rate can be obtained in case of a pressure shock.

Owing to the smaller size of the possibly included directly controlledshock valves and to fewer variants, a lower cost can be achieved.Furthermore, owing to the smaller directly controlled shock valves, thevalve housing can be made smaller.

As a rule, the control valve opens more slowly than the shock valve,which in many cases implies that said flow communication between theactuator and the tank via the first conduit is opened after the shockvalve has opened the flow communication between the actuator and thetank via the second conduit. In other words, the control valve is openedwith a certain delay, so that the shock valve is opened first. It ispossible, however, to ensure that the control valve opens substantiallysimultaneously as, or before the shock valve.

Preferably, a shock valve of a smaller size, i.e. with a lower nominalflow rate, than the electrically controlled outlet valve is used. Thedirectly controlled shock valve, which is fast-acting, opens directlyand flow drainage is initiated. Then, the electrically controlledcontrol valve, which is capable of handling the larger flow requirementand draining it to tank, is opened.

According to another preferred embodiment, the method comprises the stepof determining the flow rate to the tank on the basis of the detectedpressure. In this way, the characteristics of the shock control functioncan be determined. The opening degree of the control valve iscontrolled, for example, on the basis of the pressure change in theactuator.

Further preferred embodiments of the invention and advantages associatedtherewith are apparent from the remaining claims and the followingdescription.

BRIEF DESCRIPTION OF FIGURES

The invention will be described more closely in the following, withreference to the embodiments shown in the attached drawings, wherein

FIG. 1 shows a side view of a wheel loader, and

FIG. 2 shows a system for performing the method during operation of thewheel loader.

DETAILED DESCRIPTION

FIG. 1 shows a side view of a wheel loader 101. The wheel loader 101comprises a front vehicle section 102 and a rear vehicle section 103,said sections each comprising a frame and a pair of drive shafts 112,113. The rear vehicle section 103 comprises a driver's cab 114. Thevehicle sections 102, 103 are connected to each other in such a way thatthey can be pivoted relative to each other about a vertical axis bymeans of two actuators in the form of hydraulic cylinders 104, 105 whichare connected to the two sections. Accordingly, the hydraulic cylinders104, 105 are disposed on different sides of a centre line in thelongitudinal direction of the vehicle for steering, or turning the wheelloader 101.

The wheel loader 101 comprises an equipment 111 for handling objects ormaterial. The equipment 111 comprises a load-arm unit 106 and animplement 107 in the form of a bucket which is fitted on the load-armunit. Here, the bucket 107 is filled with material 116. A first end ofthe load-arm unit 106 is pivotally connected to the front vehiclesection 102 in order to achieve a lifting movement of the bucket. Thebucket 107 is pivotally connected to a second end of the load-arm unit106 in order to achieve a tilting movement of the bucket.

The load-arm unit 106 can be raised and lowered relative to the frontsection 102 of the vehicle by means of two actuators in the form ofhydraulic cylinders 108, 109, each of which is connected at one end tothe front vehicle section 102 and at the other end to the load-arm unit106. The bucket 107 can be tilted relative to the load-arm unit 106 bymeans of a third actuator (hydraulic cylinder) 110, which is connectedat one end to the front vehicle section 102 and at the other end to thebucket 107 via a link arm system.

A first embodiment of the system is shown in FIG. 2. The system 201comprises a pump 205 adapted to supply the hydraulic cylinders withpressurized hydraulic fluid via a hydraulic circuit. The pump 205 isdriven by the vehicles propulsion engine 206, in the form of a dieselengine. The pump 205 has a variable displacement. The pump 205 ispreferably adapted for infinitely variable control. The system 201comprises a valve device 208 (se the dash-dotted line) which comprises ahydraulic circuit having a plurality of control valves for controllingthe lift and tilt function.

Two control valves, in the form of flow valves, 207, 209, are arrangedbetween the pump 205 and the lift cylinders 108, 109 in the circuit inorder to control the lifting and lowering movement. While a first one ofthese valves 207 is arranged to connect the pump 205 to the piston side,a second one of these valves 209 is arranged to connect a tank 243 tothe piston rod side. Furthermore, the first valve 207 is arranged toconnect the tank 243 to the piston side and the second valve 208 isarranged, correspondingly, to connect the pump 205 to the piston rodside. This offers large possibilities for varying the control. Inparticular, it is not necessary to connect the pump and tanksimultaneously to the function.

The system 201 further comprises a control unit 213, or computer, whichcontains software for controlling the functions. The control unit isalso called a CPU (central processing unit) or ECM (electronic controlmodule). The control unit 213 suitably comprises a microprocessor.

An operator-controlled element 211, in the form of a lifting lever, isoperatively connected to the control unit 213. The control unit 213 isadapted to receive control signals from the control lever and to actuatethe control valves 207, 209 correspondingly (via a valve control unit215). The control unit 213 preferably controls more general controlstrategies and the control unit 215 controls basic functions of thevalve unit 208. Naturally, the control units 213, 215 can also beintegrated into a single unit. When controlling the pump 205, there isan oil flow out to the cylinders 108, 109, the level of which depends onthe extent to which the actuated valves 207, 209 are opened.

An operator-controlled element 219, in the form of a steering-wheel, ishydraulically connected to the steering cylinders 104, 105, via a valveunit in the form of an orbitrol unit 220, for direct-control thereof.

Similarly as for the lift function, two control valves 223, 225 arearranged between the pump 205 and the tilt cylinder 100 for controllingthe forward and return movement of the implement relative to theload-arm unit. An operator-controlled element 227, in the form of tiltlever, is operatively connected to the control unit 213. The controlunit 213 is adapted to receive control signals from the tilt lever andto actuate the control valves 223, 225 correspondingly.

A prioritizing valve 220 is arranged at the outlet conduit 245 from thepump in order to automatically prioritize that the steering functionreceives the required pressure before the lift function (and the tiltfunction).

The system 201 is load-sensing and comprises, for this purpose, aplurality of pressure sensors 229, 231, 233, 235, 237 for detecting loadpressures of each of said functions. The lift function of the systemcomprises two pressure sensors 229, 231, out which one is arranged on aconduit to the piston side of the lift cylinders and the other on aconduit to the piston rod side of the lift cylinders. In a correspondingway, the tilt function of the system comprises two pressure sensors 235,237, out of which one is arranged on a conduit to the piston rod side ofthe tilt cylinder and the other on a conduit to the piston side of thetilt cylinder. The steering function comprises a pressure sensor 233 ona conduit connected to the steering cylinders 104, 105. More precisely,the pressure sensor 233 is situated on the LS-conduit which receives thesame pressure as on one cylinder side when steering in one direction andas on the other cylinder side when steering in the other direction. Inneutral, the LS-conduit is connected to tank.

The system further comprises an electrically controlled valve 241adapted to control the output pressure of the pump via a hydraulicsignal. The system 201 comprises an additional pressure sensor 239 fordetecting a pressure which is indicative of an output pressure from thepump. More precisely, the pressure sensor 239 is adapted to detect thepressure in a position downstream the electrically controlled valve 241.Accordingly, the pressure sensor 239 senses the pump pressure directlywhen the valve 241 is fully open. In normal driving conditions, thepressure sensor 239 detects the output pressure from the valve 241.Accordingly, the control unit 213 is adapted to receive a signal fromthe pump pressure sensor 239 with information about of the pressurelevel.

Accordingly, the control unit 213 receives electrical signals from thepressure sensors 229, 231, 233, 235, 237, 239 and generates anelectrical signal for controlling the electrical valve 241.

As previously stated, the control unit 213 is adapted to receive signalsfrom the control levers 211, 227. When the operator wants to lift thebucket, the lift lever 211 is operated. The control unit receives acorresponding signal from the lift lever 211 and actuates the controlvalves 207, 209 to such a position that the pump is connected to thepiston side of the lift cylinders 108, 109 and the piston rod side ofthe lift cylinders is connected to the tank 243. Furthermore, thecontrol unit receives signals from the load pressure sensor 229 on thepiston side of the lift cylinders and from the pressure sensor 239downstream the pump. Based upon the received signals, a desired pumppressure at a level above the detected load pressure is determined, andthe electrically controlled pump control valve 241 is actuatedcorrespondingly.

The control unit 213 is preferably adapted to coordinate the openingdegree of the control valves 207, 209 and the output pressure of thepump 205 for optimum operation.

The tilt function is controlled in a corresponding manner as the liftfunction. When steering the machine, the pressure sensor 233 of thesteering function detects a load pressure of the steering and generatesa corresponding load signal. The control unit 213 receives this loadsignal and a signal from the pressure sensor 239 on the outlet conduitof the electrically controlled valve 241. Based upon the receivedsignals, a desired pump pressure at a level above the detected loadpressure is determined, and the electrically controlled pump controlvalve 241 is actuated correspondingly.

When several functions are used simultaneously, the detected loadpressures are compared and the pump 205 is controlled corresponding tothe highest of the detected load pressures.

Accordingly, the electrically controlled pump control valve 241 isadapted to be infinitely adjustable between two end positions, a firstend position which corresponds to the pump producing a minimum pressureand a second end position which corresponds to the pump producing amaximum pressure.

A hydraulic means 253, in the form of a reversing valve, is arranged ona conduit 251 between the electrically controlled pump control valve 241and the pump. The reversing valve 253 is adapted to receive thehydraulic signals from the steering function and the pump control valve241.

Furthermore, the reversing valve is adapted to control the pump 205corresponding to the received signal having the largest load pressure.Accordingly, the hydraulic means (reversing valve) 253 selects thehigher pressure in an output signal made up of two input pressuresignals.

The system further comprises a sensor 255 for detecting lift cylinderposition. The sensor 255 is operatively connected to the control unit213. In this way, the control unit 213 can decide whether a lifting orlowering movement of the load is performed.

The system 201 further comprises a number of shock valves 261, 263, 367,for the lift function and the tilt function, for draining hydraulicfluid to the tank 243 in case of a strong pressure increase. The liftfunction of the system comprises a shock valve 261 which is arranged ona conduit 273 to the piston side of the lift cylinders. The tiltfunction of the system comprises two shock valves 263, 267, out of whichone 263 is arranged on a conduit 277 to the piston rod side of the tiltcylinder and the other 267 on a conduit 279 to the piston side of thetilt cylinder.

Below, a method for automatically limiting a pressure generated duringoperation in the system when needed is described in a few differentexamples. The method is described with respect to the lift function, butthe corresponding also applies to, for example, the tilt function.

An external force initiates a movement of the hydraulic cylinders 108,109. The control unit 213 detects that the pressure exceeds a certainfirst level (for example 350 bar) via the pressure sensor 229. Thecontrol unit 213 then emits a signal to the outlet valve 207 to drainoil to the tank 243 via a first conduit 271. Accordingly, the outletvalve 207 acts like a shock valve by means of software control. Thedirectly controlled shock valve 261 opens when the pressure exceeds acertain second, predetermined level (for example 360 bar) and initiatesdraining of flow to the tank 243 via a second conduit 273. Theelectrically controlled outlet valve 207 now has had time to open for alarger drainage flow to the tank 243. The pressure, which is recordedcontinuously, drops and the electrically controlled outlet valve 207 andthe directly controlled shock valve 261 close at specific pressurelevels.

The first level can be equal to the second level, but preferably thefirst level is smaller than the second level. This in order to obtain asubstantially simultaneous, or earlier, opening of the control valverelative to the shock valve.

As a supplement or an alternative to the foregoing, the electricallycontrolled outlet valve 207 is controlled on the basis of the pressurederivative (in order to obtain faster opening of the electricallycontrolled outlet valve 207). For example, the control valve iscontrolled to serve as a shock valve as soon as the pressure derivativein the cylinder 108, 109 exceeds a certain level, irrespective ofwhether the pressure level is low. If an external force initiates amovement of the cylinder, the control valve will initiate its openingprocedure before the pressure level reaches the upper limit (for example350 bar). If the upper limit is not reached, the control valve willstill close when the pressure derivative falls short of a certain level.

According to a further variant, the electrically controlled shock valve207 has a variable opening pressure. Preferably, the pressure level isset depending upon the operating condition (such as load-arm positionand/or bucket position). The directly controlled shock valve 261 is thenset to open only at the maximum pressure level. In certain situations, alarge shock resistance is needed, for example when the bucket is pushedinto a material pile with maximum propulsion, and in other situations,the shock function can open at a lower pressure. This means that themachine/iron is subjected to less stress.

The opening pressure of the electrically controlled valve 207 is, forexample, dependent on the following operating parameters:

Cylinder positions for different functions. For example, when the bucketis pushed with maximum propulsion into the material pile (when the unitis lowered and the bucket is in a level position) an exceptionally highresistance is needed on the piston side of the lift cylinder.

Type of implement. Implements which are not influenced by the propulsion(for example a pallet fork assembly) do not need as high an openingpressure as a bucket.

Type of handling. One handling example is loading timber onto a truck.Another example is bucket handling for loading gravel/rocks.Furthermore, it is conceivable to use the same implement, for example abucket, for different handling operations. Accordingly, type of handlingcan be independent of type of implement.

According to one example, the system is adaptive. The control unit canthen record how the wheel loader is operated during a certain period oftime through detecting operating parameters and concluding whichhandling operation is performed and/or which implement type is used.Alternatively, or as a supplement, the limit value is determined on thebasis of a signal from an operator-controlled element, such as a lever,button, or another control means in the cab.

Machine speed. At high machine speeds, it is safer if the openingpressures of the shock valves are at a higher level.

According to a further variant, the electrically controlled valve 207has different pressure drops for the same flow rate, wherein thepressure drop is dependent on the following:—the function concernedand/or—the cylinder position. When subjected to shock loading with theload-arm in a high position, it is not desirable that the unit falls tothe ground, but is lowered at a controlled speed. With this system allfunctions and all machine sizes can have the same shock characteristics,that is to say, when the shock function opens, the same degree ofresistance can be felt irrespective of the type of machine concerned.

Furthermore, an adaptive shock control on the basis of a pressure levelcan be utilized. The basic idea is to have as low an opening pressure aspossible, with the purpose of “sparing” the machine. The machines whichare handled most aggressively are the ones which to a great extentdecide the opening levels. Therefore, according to a further variant, anadaptive opening pressure is introduced. Thereby, most of the machinescan be at lower levels and the machines which require higher levels willalso get such levels. The idea is that the control unit 213 records theextent of shock loading which occurs. If this exceeds a certain level,the opening pressure for the electrically controlled shock valve 207 istemporarily increased within certain limits. The opening pressure can bea function of all or certain of the following: shock loading frequency,shock loading time, shock loading time expressed as a percentage oftotal machine time (with diesel engine running) and/or shock loadingtime expressed as a percentage of total active time for the functionconcerned.

Similar adaptive action can also occur when the electrically controlledshock valve 207 opens at a certain pressure derivative. The pressurederivative limit can be adjusted depending upon how often/much theelectrically controlled shock valve 207 opens as a result of thepressure derivative. The same function dependent parameters as describedabove can be used, but where, as mentioned before, only those caseswhere the shock loading control occurs as a result of the pressurederivative are taken into consideration.

The invention should not be regarded as limited to the above-describedexemplary embodiments, but a number of further variants andmodifications are conceivable within the scope of the following claims.In particular, the preferred embodiments can be combined in a number ofdifferent ways.

1. Method for automatically limiting a pressure generated duringoperation in a hydraulic system when needed, system being adapted todeliver a pressurized hydraulic fluid to at least one actuator arrangedto perform a work function by means of an implement of a workingmachine, wherein the method comprises the steps of: detecting a pressurein at least one position in the system; comparing the detected pressurevalue, or a value associated with the detected pressure value, with afirst predetermined limit value; and opening a flow communicationbetween the actuator and a tank via a first conduit if the detectedpressure value, or the value associated with the detected pressurevalue/exceeds the predetermined limit value, detecting an operatingparameter which is indicative of a position of the implement anddetermining the limit value on the basis of the detected operatingparameter.
 2. Method according to claim 1, wherein the flowcommunication is opened via a control valve being arranged on the firstconduit and having the function to control the supply of the hydraulicfluid to and from, respectively, the actuator with the purpose ofperforming the work function.
 3. Method according to claim 2, comprisingthe step of opening the control valve via an electrical signal. 4.Method according to claim 1, wherein a first control valve is arrangedon a conduit connecting to a first side of the actuator and a secondcontrol valve is arranged on a conduit connecting to a second side ofthe actuator, comprising the step of detecting the pressure on least oneof these actuator sides and opening the control valve (207, 223) totank, which is arranged on the side where increased pressure has beengenerated.
 5. Method according to claim 1, further comprising the stepof opening a flow communication between the actuator and the tank via asecond conduit via a shock valve.
 6. Method according to claim 5,wherein the shock valve is passive.
 7. Method according to claim 5,wherein the shock valve is spring-loaded,
 8. Method according to claim5, wherein the first and second conduit are connected to the same sideof the actuator,
 9. Method according to claim 2, comprising opening aflow communication between the actuator and the tank via a secondconduit is a shock valve, wherein the control valve drains a larger flowto tank than the shock valve does.
 10. Method according to claim 1,further comprising the steps of detecting the pressure when flowcommunication between the actuator and the tank via the first conduithas been opened, and closing flow communication between the actuator andthe tank via the fist conduit if the pressure value, or the valueassociated with the detected pressure value, falls short of a secondpredetermined limit value being lower than the first limit value. 11.Method according to claim 1, comprising the step of detecting thepressure of the actuator.
 12. Method according to claim 1, comprisingthe step of detecting a level of the pressure in position in the system,comparing the pressure level with a first predetermined limit value forthe pressure level, and opening the flow communication between theactuator and the tank via the first conduit if the pressure levelexceeds the predetermined limit value.
 13. Method according to claim 1,comprising the step of determining a derivative of the pressure inposition in the system, comparing the pressure derivative with a firstpredetermined limit value for the pressure derivative, and opening theflow communication between the actuator and the tank via the firstconduit if the pressure derivative exceeds the predetermined limitvalue,
 14. Method according to claim 1, comprising the step ofdetermining the limit value on the basis of the actual operatingcondition,
 15. Method according to claim 1, comprising the step ofdetecting at least one operating parameter and determining the limitvalue on the basis of the detected operating parameter.
 16. Methodaccording to claim 1, comprising the step of detecting at least oneoperating parameter repeatedly and determining the limit value basedupon how the work function is performed.
 17. Method according to claim1, comprising the steps of controlling a plurality of work functions,including lifting and tilting of an implement.
 18. Method according toclaim 1, wherein different limit values are associated with at least twoof the work functions, wherein the method comprises the step ofselecting the limit value which is associated with the work functionbeing performed, for comparison.
 19. Method according to claim 1,comprising the steps of controlling a working machine, working machinecomprising system.
 20. Method according to claim 19, comprising the stepof detecting an operating parameter which is indicative of the type ofimplement being actuated via the actuator and determining the limitvalue on the basis of the detected implement type.
 21. Method accordingto claim 19, comprising the step of detecting an operating parameterwhich is indicative of the type of handling being performed with themachine and determining the limit value on the basis of the detectedtype of handling operation.
 22. Method according to claim 20, comprisingthe step of detecting an operating parameter which is indicative of amachine speed and determining the limit value on the basis of thedetected machine speed.
 23. Method according to claim 1, comprising thestep of determining the flow rate to the tank on the basis of thedetected pressure.